Quoting a 2020/01/06 The Cinemaholic article:
In 2017, director Shin Won-Ho, when asked about the possibility of Season 4 in a press conference, answered that he has not been working on it as he has been busy with other projects. However, he suggested that he continues to research and hunt for the right story.
As of today, there is no official news of a ‘Reply’ Season 4. However, with the director looking for a new tale, ‘Reply’ Season 4 might get a release date sometime in 2022.
If they will remain faithful to the series' trend, then Reply 1972 will be more likely to happen. However, it's quite a bleak time in Korean history since it was during that year that the Martial Law was declared. I could not imagine how they would be able to pull off some comedic scenes in such an era. Would it be like The Light In Your Eyes?
2002 is a good year though. It won't be bad to watch a Reply drama set during that time.
Meanwhile here's a funny clip of a 1970s scene from The Light In Your Eyes: